In a conventional thread clamp of this type (known from German Pat. No. 813,978), the clamping force by which each jaw is urged against the abutment bar can be adjusted by means of a screw or nut, which acts onto the spring. However, this clamping force remains constant during the entire tying operation. This results in a damage to the threads when the apparatus is handling delicate threads, for example Boucle or Effect threads. During the tying operation the clamped threads are pulled by the tying needle into the tying arbor and are caused to slide thereby between the clamping jaws and the abutment bar. Since Boucle and Effect threads have different thicknesses due to the loops and knots, this causes a variable clamping force to occur, which may be sufficient that the loops are pulled out or, in addition, a thread breakage can occur if a knot in the thread becomes caught in the thread clamp. Also when the threads are introduced into the closed thread clamp, difficulty and damage may occur due to the different thickness of the threads. It is not possible to overcome these deficiencies by reducing the spring force because the threads are then no longer held with a sufficient clamping force during a part of the tying operation.
Therefore the basic purpose of the invention is to produce a thread clamp for warp end tying-in machines of the above-mentioned type, which avoids damage to the threads which are to be tied and which is suited in particular for the holding of Boucle and Effect threads.